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Implement LWG-3549 #2198

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Merged
Stephan T. Lavavej (StephanTLavavej) merged 1 commit into from
Sep 25, 2021
Merged

Implement LWG-3549 #2198

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17 changes: 15 additions & 2 deletions stl/inc/xutility
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Original file line number Diff line number Diff line change
Expand Up @@ -2681,8 +2681,21 @@ namespace ranges {

struct view_base {};

template <template <class...> class _Template, class... _Args>
void _Derived_from_specialization_impl(const _Template<_Args...>&);

template <class _Ty, template <class...> class _Template>
concept _Derived_from_specialization_of = requires(const _Ty& _Obj) {
_Derived_from_specialization_impl<_Template>(_Obj);
};

template <class _Derived>
requires is_class_v<_Derived> && same_as<_Derived, remove_cv_t<_Derived>>
class view_interface;

template <class _Ty>
inline constexpr bool enable_view = derived_from<_Ty, view_base>;
inline constexpr bool enable_view =
derived_from<_Ty, view_base> || _Derived_from_specialization_of<_Ty, view_interface>;

#ifdef __cpp_lib_ranges // TRANSITION, GH-1814
template <class _Ty>
Expand Down Expand Up @@ -3016,7 +3029,7 @@ namespace ranges {
// clang-format off
template <class _Derived>
requires is_class_v<_Derived> && same_as<_Derived, remove_cv_t<_Derived>>
class view_interface : public view_base {
class view_interface {
private:
_NODISCARD constexpr _Derived& _Cast() noexcept {
static_assert(derived_from<_Derived, view_interface>,
Expand Down
29 changes: 28 additions & 1 deletion tests/std/tests/P0896R4_ranges_range_machinery/test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1578,6 +1578,7 @@ using immutable_badsized_range = badsized_range<immutable_sized_range>;
template <class T>
constexpr bool ranges::disable_sized_range<badsized_range<T>> = true;

// "strange" in that const-ness affects the iterator type
struct strange_view {
strange_view() = default;
strange_view(strange_view&&) = default;
Expand All @@ -1591,7 +1592,15 @@ struct strange_view {
};

struct strange_view2 : strange_view, ranges::view_base {};
struct strange_view3 : strange_view2 {};
struct strange_view3 : strange_view2 {}; // not truly a view since enable_view<strange_view3> is false (see below)
struct strange_view4 : strange_view, ranges::view_interface<strange_view4> {};
struct strange_view5 : strange_view4, ranges::view_interface<strange_view5> {
// not truly a view since enable_view<strange_view5> is false due to multiple inheritance from view_interface
};

// Verify that specializations of view_interface do not inherit from view_base
STATIC_ASSERT(!std::is_base_of_v<std::ranges::view_base, ranges::view_interface<strange_view4>>);
STATIC_ASSERT(!std::is_base_of_v<std::ranges::view_base, ranges::view_interface<strange_view5>>);

template <>
inline constexpr bool ranges::enable_view<strange_view> = true;
Expand Down Expand Up @@ -1620,6 +1629,7 @@ namespace exhaustive_size_and_view_test {

using I = int*;
using CI = int const*;
using D = std::ptrdiff_t;
using S = std::size_t;
using UC = unsigned char;

Expand Down Expand Up @@ -1682,6 +1692,23 @@ namespace exhaustive_size_and_view_test {
STATIC_ASSERT(test<strange_view3&, false, I, S>());
STATIC_ASSERT(test<strange_view3 const, false, CI, S>());
STATIC_ASSERT(test<strange_view3 const&, false, CI, S>());

STATIC_ASSERT(test<strange_view4, true, I, D>());
STATIC_ASSERT(test<strange_view4&, false, I, D>());
STATIC_ASSERT(test<strange_view4 const, false, CI, D>());
STATIC_ASSERT(test<strange_view4 const&, false, CI, D>());

template <class = void>
constexpr bool strict_test_case() {
if constexpr (!is_permissive) {
STATIC_ASSERT(test<strange_view5, false, I, S>());
}
return true;
}
STATIC_ASSERT(strict_test_case());
STATIC_ASSERT(test<strange_view5&, false, I, S>());
STATIC_ASSERT(test<strange_view5 const, false, CI, S>());
STATIC_ASSERT(test<strange_view5 const&, false, CI, S>());
} // namespace exhaustive_size_and_view_test

// Validate output_range
Expand Down